L15. Rockets and Satellites: Getting Off Earth

Unit 3 · Gravity and Motion

R-report

L15. Rockets and Satellites: Getting Off Earth

A rocket has nothing to push against. There is no air in space, no ground, no water — and yet it accelerates. Understanding how is the difference between believing space travel is magic and knowing it is physics.

Push something back, and you go forward

Rockets work by Newton’s third law: for every action there is an equal and opposite reaction. A rocket burns fuel and throws the resulting hot gas out of its nozzle at enormous speed. Throwing that mass backward pushes the rocket forward. It does not need anything outside itself to push against — which is exactly why rockets work in the vacuum of space, and why they work better there than in air. You can demonstrate the principle in a corridor. Inflate a balloon and let it go: escaping air rushes one way, the balloon flies the other. Or stand on a skateboard and throw a heavy bag forward; you roll backward. The hard part is not the principle but the quantity. To reach orbit a rocket must accelerate to about 28,000 km/h while carrying its own fuel and lifting against gravity and air resistance. The fuel needed to lift the fuel is the central engineering problem of spaceflight.

Why rockets come apart on the way up

Watch a launch and you will see the rocket discard large sections during the climb. That is staging, and it solves the fuel problem. As fuel burns, the tanks holding it become dead weight. Carrying empty tanks all the way to orbit wastes energy, so a staged rocket drops each section as it empties. - The first stage is the largest. It lifts everything off the pad through the thickest air, then separates. - The second stage takes over in thin air and pushes the vehicle to orbital speed. - The payload — a satellite, probe, or crew capsule — is what remains at the top. Only a small fraction of the mass on the launch pad ever reaches orbit. For many rockets, the payload is around 2 to 4% of the total lift-off mass; the rest is fuel and structure. Modern designs increasingly recover the first stage and fly it again, which cuts cost substantially. That change, more than any new physics, is what has made launches more frequent in recent years.

Choosing an orbit for the job

Once in space, the height of the orbit determines what a satellite can do. - Low Earth orbit (about 200 to 2,000 km): the space station, Earth-observation satellites, and most communications constellations. Close enough for detailed imaging and low signal delay, and one lap takes 90 to 120 minutes. - Polar orbit: passes near both poles while Earth rotates underneath, so over time it scans the whole planet strip by strip. Ideal for weather and mapping. - Geostationary orbit (35,786 km above the equator): one orbit per day, so the satellite hangs over the same spot. Used for television, some weather satellites, and communications relays. To leave Earth entirely, a spacecraft must reach escape speed, about 40,000 km/h. There is a growing problem to be aware of: space debris. Old satellites, spent stages, and fragments now number in the millions, and at orbital speeds even a paint fleck can damage a spacecraft. Keeping low orbit usable is becoming a serious engineering and policy issue.

Putting the investigation together

Rockets move by throwing mass backward, which works better in vacuum than in air. Getting to orbit means reaching about 28,000 km/h, and staging exists because carrying empty fuel tanks that fast is wasteful. Where a satellite ends up then decides what job it can do.

It is worth noticing that spaceflight required no new physics — Newton’s laws were three centuries old before anything reached orbit. What was missing was engineering: materials, fuels, guidance, and control. Understanding a principle and being able to build with it are different achievements.